The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

Obsoletes the same variable from AudioConfig.cxx.

Move code from replay_gain_get_real_mode().

Replaces the old BoundMethod template.

All remaining events are specific to the Partition.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

Necessary for "final" on gcc 4.6.

Use a listener interface instead of GlobalEvents.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Don't use global variable.

Another big chunk of code for multi-player support.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.